Interior acoustic noise is a primary concern in the operation of helicopters. While there are numerous sources of acoustic noise-generating vibrations in an operating helicopter, such as the main rotor assembly, the main gearbox, the engines, the tail rotor assembly, the hydraulic system, aerodynamic forces, etc., the high frequency structure-borne vibrations emanating from the main gearbox have the most pronounced effect on interior acoustic noise, i.e., in the cockpit and/or cabin.
In a Sikorsky Aircraft Corporation S-92.TM. helicopter (S-92.TM. is a trademark of the Sikorsky Aircraft Corporation), the main gearbox includes three stages of reduction gearing: a first stage for each engine output comprising input and output bevel gearing, a second stage comprising two driver bevel pinions driving a main bevel gear, and a final stage comprising a stacked compound planetary gear train having a plurality of primary planetary pinions interacting with a sun gear, and a plurality of secondary planetary pinions interacting with a fixed ring gear (a more detailed description of the operation of the S-92 helicopter's main gearbox can be found in U.S. Pat. No. 5,472,386, STACKED COMPOUND PLANETARY GEAR TRAIN FOR AN UPGRADED POWERTRAIN SYSTEM FOR A HELICOPTER, granted to Kish, and assigned to United Technologies Corporation).
The high frequency vibrations emanating from the main gearbox are coupled to the helicopter airframe structure via main gearbox support members, and induce vibratory responses of many airframe structure natural modes. These vibratory responses excite acoustic natural modes of the cockpit and/or cabin acoustic volume and produce undesirable acoustic noise levels within the helicopter cockpit and/or cabin.
In normal operations, dominant cockpit and/or cabin acoustic noise levels of the S-92 helicopter are primarily the result of high frequency vibrations originating from gear meshing between the secondary planetary pinions and the fixed ring gear in the stacked compound planetary gear train. As illustrated in FIG. 1, the vibrations produced by the first and second reduction stages of the S-92 helicopter's main gearbox, and the vibrations produced by the gear meshing between the primary planetary pinions and the sun gear, occur at very high frequencies 2, 4A, 4B (greater than 1000 Hz), and generate acoustic noise in the cabin and/or cockpit that is minor relative to acoustic noise generated by the gear meshing between the secondary planetary pinions and the fixed ring gear (which occurs at a fundamental frequency 6 of approximately 687.7 Hz at 100% Nr, and can vary between approximately 618.9 Hz at 90% Nr and approximately 722.1 Hz at 105% Nr). Specifically, the high frequency vibrations produced by the gear meshing between the secondary planetary pinions and the fixed ring gear generate acoustic noise in the cabin and/or cockpit that fall into the speech interference range, thereby making them undesirable.
Such acoustic noise generally cannot be effectively abated by passive-type acoustic treatment of the cockpit and/or cabin interior. Passive treatment, such as acoustic panels or blankets, may be partially effective for very high frequency induced acoustic noise, but are not very effective vis-a-vis induced acoustic noise in the 300 to 1000 Hz range. In addition, the weight penalty incurred by the use of such acoustic panels or blankets negatively impacts the performance capability of the helicopter.
Another passive technique involves the use of vibration isolators at the interface between the main rotor assembly/main gearbox and the airframe structure. Such vibration isolators transmit only a reduced portion of the acoustic noise-generating high frequency vibrations into the helicopter airframe due to their inherent softness. These vibration isolators, however, must be interposed in the primary load path of the helicopter, and gearbox deflections under steady flight loads may cause high speed engine-to-transmission drive shaft deflections that may adversely impact shaft reliability and could also induce false commands into the flight control system.
In U.S. Pat. No. 5,310,137, HELICOPTER ACTIVE NOISE CONTROL SYSTEM, granted to Yoerkie et al., and assigned to United Technologies Corporation (hereinafter "'137 patent"), an active noise control system for a helicopter is disclosed that is operative to effectively nullify one or more high frequency vibrations emanating from a gearbox at a gearbox/airframe interface, thereby significantly reducing the interior noise levels of the helicopter. The active noise control system is design optimized to minimize the number of actuators required, and is design optimized to minimize contamination forces arising from operation of the system actuators. The active noise control system includes modified transmission beams that are mechanically stiffened to function as rigid bodies with respect to the one or more of the high frequency vibrations, a plurality of actuators disposed in combination with the modified transmission beams, a plurality of sensors disposed in combination with the modified transmission beams in a collinear, spaced apart functional correlation with respective actuators, and controllers interconnecting individual actuators with respective functionally correlated sensors.
A drawback to the active noise control system disclosed in the '137 patent is that although the placement of the actuators and sensors on the transmission beams results in localized nullification of high frequency vibrations at the sensor locations, the location of the sensors and actuators remotely from the gearbox/airframe interface may permit the "leaking" of high frequency vibrations into the helicopter's airframe through the space between the gearbox/airframe interface and the sensor locations. Therefore, although the sensors may return data to the controller indicative of nullified high frequency vibrations, there still exists a possibility that undesirable acoustic noise is being generated in the cabin.